A method of the specified type is used for the sinking of wells which have long been known as “horizontal filter wells”. The Fehlmann and Preussag methods are often used to sink these wells. These methods are described in E. Bieske “Bohrbrunnen” (7th ed., 1992, Oldenbourg, Munich, pages 19 to 23.). Both methods involve first the sinking of a perpendicular shaft which reaches down to the aquifer and serves as a starting shaft for driving the horizontal holding strings and, once the holding strings have been completed, is developed into a pump shaft. The shaft construction generally consists in this case of reinforced concrete pipes having an internal diameter of 2.0 m or more, placed one on top of another. The pipes are laid with the aid of hydraulic presses or a superimposed load. The soil infiltrating the pipes is removed. Once the desired depth has been reached, the bottom of the shaft is covered with concrete. Starting from the shaft, horizontal bores are then driven using drill pipes through openings in the shaft wall. Filter pipes are then introduced into these drill pipes before the drill pipes are removed. In the Preussag method, the filter pipes are additionally surrounded by a gravel envelope.
In the known methods, the drill pipes are driven based on the displacement principle by advancing the drill pipes while at the same time removing fine grain. The starter drill pipe is given a conical drill head which penetrates the subsoil during driving of the drill pipes. The drill head has a large number of suitably sized holes. By constantly moving the entire string of pipes, including the drill head, back and forth, the hydrostatic pressure of the groundwater pushes the drilled material into the drill head. A separating plate closes off the drill head from the inside of the drill pipes. Screwed into the separating plate are return rods through which the drilled material and water infiltrating the drill head are conveyed out toward the shaft. In difficult soil conditions, in particular in cohesive or compact formations, the transportation of the drilled material and also the loosening of the soil are assisted by additional flushing with pressurized water. The pressurized water is led through a separate flushing pipe, installed in the string of pipes, to the drill head and issues within the drill head. Once the intended string length has been reached, the return rods and the flushing rods are unscrewed from the partition between the drill head and the first drill pipe and withdrawn toward the shaft, the partition being sealed by a self-closing flap. The drill pipe, which is sealed toward the rock mass, is then available for installation of the filter pipes. Once the filter pipes have been installed and, in the Preussag method, a gravel packing introduced, the drill pipes are gradually withdrawn into the shaft. The drill head is left behind in the rock mass and lost.
The known methods have proven successful in practice. However, they can be used only in soil formations in which the drill head can be advanced and freely flushed. Stones and deposits of clay can constitute insuperable obstacles to drilling in these methods and rock formations cannot be drilled.
Also known from DE 100 29 476 A1 is a drilling device with which, starting from a start pit, a drilling device and subsequently product pipes can be driven in the horizontal direction by means of a hydraulic press unit. The drilling device comprises a shield in which a drive shaft carrying a tool disc is rotatably mounted and can be driven by a motor. Arranged after the tool disc is a cell wall comprising cells which receive drilled material removed by the tool disc. A conveying pipe which is arranged after the cell wall and has a receiving end facing the cell wall can be moved past the cells and conveys the drilled material contained in the cells successively through the product pipes and out of the start pit. The material can be conveyed with the aid of air or water which is led with excess pressure via a further pipe into the cells. This device also has the drawback that the drill head cannot be withdrawn along with the tool disc and cell wall, after completion of a string bore, through the production pipes but must rather be left behind in the string bore as a lost component of the device. Also, if the drilling device becomes damaged or blocked, it is almost impossible to carry out repairs, so it may not be possible to continue the drilling drive operation.
Also known from DE 28 29 834 is a method for drilling a bore hole in a subsoil permeated with boulders or layers of rock using a ground drilling device which consists of a cylindrical drill casing and a rock drill bit and in which the rock drill bit is introduced, with cutting tools drawn into their inner position, into the drill casing in such a way that the movable cutting tools are located below the lower end of the drill casing. Subsequently, the movable cutting tools are moved into their outer cutting position and the rock drill bit is lowered together with and at the same time as the drill casing and they are set in rotation about their common axis to drill a bore hole, the diameter of which is at least equal to the outer diameter of the drill casing. Once the drilling process has been completed, the movable cutting tools are drawn back in, so the rock drill bit can be extracted from the drill casing.